This invention relates to induction logging techniques and, more particularly, to induction logging apparatus and method for obtaining measurements dependent upon the vertical conductivity component of formations surrounding a borehole.
Conventional induction well logging techniques employ coils wound on an insulating mandrel. One or more transmitter coils are energized by an alternating current. The oscillating magnetic field produced by this arrangement results in the induction of currents in the formations which are nearly proportional to the conductivity of the formations. These currents, in turn, contribute to the voltage induced in one or more receiver coils. By selecting only the voltage component which is in phase with the transmitter current, a signal is obtained that is approximately proportional to the formation conductivity. In conventional induction logging apparatus, the basic transmitter coil and receiver coil has axes which are aligned with the longitudinal axis of the well logging device. (For simplicity of explanation, it will be assumed that the borehole axis is aligned with the axis of the logging device, and that these are both in the vertical direction. Also single coils will subsequently be referred to without regard for focusing coils or the like.) This arrangement tends to induce secondary current loops in the formations which are concentric with the vertically oriented transmitting and receiving coils. The resultant conductivity measurements are indicative of the horizontal conductivity (or resistivity) of the surrounding formations. There are, however, various formations encountered in well logging which have a conductivity that is anisotropic. Anisotropy results from the manner in which formation beds were deposited by nature. For example, "uniaxial anisotropy" is characterized by a difference between the horizontal conductivity, .sigma..sub.H, in a plane parallel to the bedding plane, and the vertical conductivity, .sigma..sub.V, in a direction perpendicular to the bedding plane. When there is no bedding dip, .sigma..sub.H can be considered to be in the plane perpendicular to the borehole, and .sigma..sub.V in the direction parallel to the borehole. The coefficient of anisotropy, designated .gamma., is defined by ##EQU1## Conventional induction logging devices, which tend to be sensitive only to the horizontal conductivity of the formations, do not provide a measure of vertical conductivity or of anisotropy.
There have been proposed in the prior art so-called "induction dipmeter" devices for measuring formation dip and/or anisotropy parameters. For example, in the U.S. Pat. No. 3,808,520, a vertical transmitter coil is used in conjunction with three receiver coils having mutually orthogonal axes; i.e., one vertical and two mutually orthogonal horizontal coils. The outputs of the three receiver coils are utilized in specified relationships to obtain combined dip and anisotropy information. According to the technique of this patent, anisotropy can be obtained if one has a knowledge of formation dip from another device, such as a conventional dipmeter device. In the U.S. Pat. No. 3,510,757, vertical transmitter coils are used in conjunction with a pair of orthogonal horizontal receiver coils, and the outputs of the receiver coils are recorded and utilized to obtain indications of formation dip. A type of prior art induction dipmeter has also been disclosed wherein a coil array is mechanically rotated at a constant frequency to produce modulation components in receiver coil signals at the frequency of rotation of the coil array. These modulation components are processed to obtain indications of dip and/or dip azimuth of formation bedding planes. See, for example, U.S. Pat. Nos. 3,014,177, 3,187,252 and 3,561,007. It is generally recognized that a disadvantage of a rotating type of induction dipmeter is the requirement for bulky and power consuming equipment for rotating coils and keeping track of their orientation.
Another typical situation where formations have conductivity characteristics that may be difficult to interpret is when heterogeneities exist. An example of a formation heterogeneity is when borehole fluids invade the formations to form a so-called invaded zone which has a different conductivity than the uninvaded formations. Another example is where a fracture in the formations is filled with a conductive fluid. In the U.S. Pat. No. 3,706,025, there is disclosed a technique whereby the resistive and reactive components of an induction logging device can be combined in a specified manner to obtain a complex conductivity which provides quantitative indication of the degree to which the formation is heterogeneous.
It is among the objects of the present invention to provide a technique for measuring the vertical conductivity of formations surrounding a borehole.